Blender IK Constraint Location Offset?
Here the file:





I simply did these steps:

1) In your Hand.L I delete the Copy Location (it's useless) and delete your IK constraint connected with the bone Hand.R

2) I create a new bone(the one in the immagine called CHILD), named BONETARGET.L (no deform), and i set this bone near to the position where i would like to have my Hand.L, hanging my weapon.

3) I create a constraint between "body" (there is a bone you called body) and BONETARGET.L; theh my BONETARGET is the child of body



4) I create an IK contraint between my BONETARGET and the Hand.L; then my Hand.L has the BONETARGET like the target for its IK



In other words i have created a chain,


you move the Hand.R that moves in the same way the "body"
the BONETARGET.L will move like the "body", and at the same time will be the target for your IK left arm that will move following this bone (BONETARGET.L)


In the end I add other 2 things:



4) I tick "Perserve volume" to mantain the volume of your mesh while you are animating, and i add a modifier (PS this one is available from the last blender verison, 2.77) called Corrective Smooth. It helps a lot and keep the mesh clear from strange and tricky deformation.

Tell me if this is the solution you were searching